1. Field of the Invention
The present invention relates to a device for measuring a rotation angle of a rotary element, and more particularly, to a rotation angle measuring device employing two circular arrays of signaling means arranged coaxially around a center of rotation and three detection means for detecting the signaling means.
2. Description of the Prior Art
As a typical construction of a rotation angle sensor, it is known to prepare a shutter disk adapted to rotate in accordance with a rotation of a rotary element of which a rotation angle is to be measured, the shutter disk having a circular array of slits centered at the rotation axis of the disk, while a light emitter and a light receiver are arranged on opposite sides of the shutter disk to align with the circular array of the slits, so that a series of electrical pulses are obtained according to a rotation of the shutter disk. In such a construction, when two sets of light emitters and light receivers are provided with a relative shifting including a fraction of the pitch of the array of the slits therealong, it is possible to detect the rotation of the shutter disk, with an identification of either of two opposite directions of rotation according to an identification of either of two different relative shiftings in the occurrence of the detection of light by the two light receivers.
When such a photo-electric sensor composed of a light emitter and a light receiver to operate with an array of slits of a shutter disk has been developed to be of some semiconductor constructions including a light emitting diode and a photo transistor, there arose a problem that the measurement of a rotation angle based upon the counting of electric pulses generated in the photo transistor due to irradiation and non irradiation of the photo transistor to the light emitted from the light emitting diode through the slits of the shutter disk is often liable to an error due to electrical noises induced in either or both of the light emitting diode and the photo transistor. If such errors in the counting due to electrical noises are left uncorrected, the errors are accumulated so much that the measurement is rendered useless. In Japanese Patent Laid-open Publication 9-113260, it is described that if such a counting error occurs in a rotation angle sensor for a steering shaft of a vehicle, the error is corrected each time when the steering shaft was turned beyond a predetermined limit angle.